The invention relates to an alternating-current filter circuit arrangement for static converter systems and a method for filtering harmonics in power-supply networks.
A previously known alternating-current filter circuit arrangement is disclosed in Austrian Pat. No. 329,677. There, one high-pulse filter tuned to lower-order harmonics and one high-pulse filter tuned to higher-order harmonics each are connected between each phase conductor of a three-phase power system in order to attenuate harmonic currents originating from a static converter circuit having a three-phase static-converter bridge. No provision is made for switching high-pulse filters on or off in dependence on reactive-power demand or on changes in the harmonics content.
Static converter systems generate a plurality of current harmonics which can give rise to undesirable distortions of the alternating or three-phase voltage and to telephone interference. In order to prevent this, tuned and/or wide-band filters are usually connected to the three-phase power rail.
A second peculiarity of a static converter is its requirement for fundamental-frequency reactive power. This is dependent on the effective power transmitted and on the terminal voltage ratio of the static-converter reactifiers. As a rule, the fundamental-frequency reactive power is made available wholly or partially by suitable compensating devices. It is possible to use several three-phase filter circuits tuned to different frequencies for this purpose and, if necessary, additional capacitor banks which are connected in parallel with the static converter. These compensating devices are switched in steps in order to match the total compensating power to the reactive-power requirement of the static converter.
As a result of this dual function of the three-phase filter circuits, the problem arises that, for example, when operating a tight high-voltage direct-current (HVDC)-transmission coupling with partial load, on the one hand the amplitudes of some harmonics increase and, on the other hand, it becomes necessary to switch off tuned filter circuits for reasons of the reactive-power balance. Especially in the lower partial-load range of an HVDC-transmission coupling (below 30% of nominal load), the problem of conducting the operation with partially disconnected filters and increased firing angle arises. As a rule, the increase in firing angle in this load range causes an increase in the current harmonics of the order n=23, 25, 35 and 37 fed into the supply network. These order numbers apply to a twelve-pulse HVDC-transmission static converter circuit.
Acting in conjunction with the above-mentioned loss in filter action if the partial-load range, system perturbations and telephone interferences can occur in this operating mode which decisively exceed the usual limits of permissible individual distortions, the total distortion and telephone interference factors.